Thermoplastic vulcanicates (TPV) are elastomer/thermoplastic polymer blends in which elastomer phase is cross-linked during the blending process and dispersed in a thermoplastic matrix. Typical TPVs are made from crosslinked EPDM dispersed in a matrix of polypropylene.
Key properties of TPVs include (1) resistance to plastic deformation (that is, low compression and tensile set), (2) resistance to fluids, (3) retention of properties at elevated temperature relative to those at ambient temperature, and (4) low creep and stress relaxation.
TPVs are prepared by a process of dynamic vulcanization, defined as “the process of intimate melt mixing of a rubbery polymer, a curative, and a thermoplastic polymer to vulcanize (or cross-link) the rubbery polymer. Upon melt mixing of the thermoplastic and rubbery polymer under high shear, the rubbery polymer tends to become the continuous phase, and as the rubbery polymer is vulcanized, a phase inversion occurs, wherein the thermoplastic material will tend to become the continuous phase with the crosslinked rubbery material dispersed in it. Various curing agents, generally known to those skilled in the art, may be employed to crosslink the rubbery component of TPVs.
The most common polymer system in TPVs is PP/EPDM rubber; however, a number of other polymer systems have been used commercially. These include PP/NBR, PP/butyl and PP/halobutyl, PP/NR, and PP/EVA/EPDM.
The morphology of a TPV is best understood as a dispersion of small and highly cross-linked elastomer particles in a continuous phase of hard thermoplastic. The size of the elastomer phase particles is one key to the performance of the TPVs. As the size of these particles decreases, the ultimate tensile properties of the TPVs increase. The second key to TPV performance is the cross-linking of the soft elastomer phase. Cross-linked elastomeric phase provides acceptable properties, which allows the TPV to capture many of the applications previously devoted to thermoset rubbers.
TPVs have various mechanical properties such as modulus (compression or tensile), tear strength, abrasion resistance and compression set resistance, which make them suited for a broad range of rubber applications.
Despite the improved properties of currently available TPVs, the dynamic vulcanization process producing such TPVs is expensive and complex because the process steps of mixing and vulcanization compete with each other and are difficult to control. Therefore, there is a need for non-vulcanized thermoplastic compositions having acceptable mechanical properties such as modulus (compression or tensile), tear strength, abrasion resistance and compression set resistance, which make such thermoplastic compositions suited for broad range of rubber applications.